1. Field of the Invention
The present invention relates to an organic EL display device, and more particularly to a technique which achieves the high definition of a display screen by reducing a size of each pixel.
2. Description of Related Art
Conventionally, a main stream of a display device has been a cathode ray tube. However, a liquid crystal display device, a plasma display device or the like which constitutes a flat display device has been put into practice replacing such a cathode ray tube, and a demand for the latter display devices is increasing. In addition to these display devices, the development and efforts for practical use of a display device using organic electro luminescence (hereinafter, referred to as an organic EL display device), and a display device which forms an image by arranging electron sources using field emission in a matrix array and by making phosphors arranged on an anode emit light (hereinafter, referred to as an FED display device) have been also in progress.
The organic EL display device possesses many advantageous features including following features.
(1) The organic EL display device is a self-luminous-type display device different from a liquid crystal display device and hence, the organic EL display device requires no backlight.
(2) A voltage necessary for emission of light is low, that is, 10V or less and hence, the power consumption can be reduced.
(3) The organic EL display device requires no vacuum structure different from the plasma display device and the FED display device and hence, the organic EL display device is suitable for achieving the reduction of weight and the reduction of thickness.
(4) A response time is short, that is, several microseconds and hence, the organic EL display device exhibits the excellent moving image characteristic.
(5) The organic EL display device possesses a wide viewing angle of 170° or more.
In the organic EL display device, an organic EL layer is formed for every pixel, and an electric current which flows in the organic EL layer is controlled by a pixel circuit. The pixel circuit requires a plurality of thin film transistors (TFT) for controlling the electric current which flows in the organic EL layer, a storage capacitance and the like. Such thin film transistors TFT include a drive TFT for controlling an electric current which flows in the organic EL layer, a switching TFT and the like. Further, the thin film transistors TFT exhibit large irregularities in threshold voltage due to irregularities in a manufacturing step thereof. When the irregularities in threshold voltage of the thin film transistor TFT is large, it is impossible to perform an accurate gray-scale control of an image. Another thin film transistor or the like becomes necessary for suppressing the irregularities of the threshold voltage of the thin film transistor TFT. In this manner, each pixel of the organic EL display device requires a large number of circuit elements therein.
The above-mentioned drive circuit is disclosed in U.S. Pat. No. 6,876,345 (patent document 1), for example.